1. Field of the Invention
The present invention relates to a disk array system used as an external storage device in a computer system and particularly relates to a disk array system in which a switching procedure for selecting a higher-performance disk array mode can be simplified.
2. Description of the Related Art
In a computer system, the opportunity of using a disk array equipment constituted by a plurality of disk devices connected in the form of an array has increased. Such a disk array equipment has been disclosed in JP-A-1-250128 or the like. This equipment has the double features of (1) high speed obtained by the simultaneous multiplex operation of a plurality of disks and (2) high reliability obtained by the existence of redundant disks.
Access control peculiar to the disk array contains: data striping for distributing data into a plurality of disks; parity generation for generating redundant data to be stored in redundant disks; and so on. To perform the access control, various methods have been proposed. The methods are roughly classified into two control methods, that is, one method for performing access control on a host computer in terms of software and the other method using disk array control hardware for performing access control in terms of hardware. In addition, a control method for performing only a part of access control such as parity generation in terms of hardware has been considered as an intermediate control method between the two methods. Software control is generally low in cost but relatively low in performance because of the increase of overhead on the host computer. On the contrary, hardware control is high in cost but is characterized in that relatively high performance can be achieved. Accordingly, by replacing software control with hardware control, an improvement in performance can be achieved at the sacrifice of increase in hardware cost.
Further, methods for connecting the disk devices to the host computer are classified into two types, that is, one type for series-connecting the disk devices onto one disk interface (hereinafter referred to as "IF") line and the other type for parallel-connecting the disk devices to the host computer through IFs separately provided for the respective disk devices. In the comparison between the two types, in the case of series-connection through one line, there arises an advantage in that the host computer requires only one connector, but performance is relatively low because only one cable is used for data transfer. In the case of parallel-connection, the number of connectors required for the host computer increases but performance is improved because the plurality of disks are used independently of each other for data transfer.
Conventionally, in the case where the disk array access control method is required to be switched, if the method for connecting the disk devices to the host computer is required to be switched in accordance with the switching of the disk array access control method, the IF board is exchanged to a new one and at the same time the connection cable is really exchanged to a new one so that the connection method is switched.
In the prior art, there is no consideration about simplification of the procedure for switching the disk array access control mode from a software control mode to a hardware control mode. That is, when, for example, the disk array access control mode is to be switched from a software control mode to a hardware control mode, it is necessary to exchange disk IF boards. There is however no consideration about difference between respective data structures of the disk IF boards depending on the difference between access control modes of the disk array, or the like. Accordingly, when the control mode is to be switched, it is necessary that disks to be used are formatted newly to reconstruct data for the disk devices.
Further, in the case where the access control mode is to be switched, control software such as an operating system and a device driver provided on the host computer must be changed simultaneously with the exchange of the IF board to a new one. Conventionally, it is however necessary that an user instructs the software to set the mode in accordance with the new IF board.
Further, because the IF board is connected to a disk device through a cable, the connection of the cable is required to be altered simultaneously with the exchange of the IF board. Further, in the case where the disk device connection is to be switched from series-connection to parallel-connection, a very troublesome procedure is required for changing the connection of the connection cable.